In recent years, remarkable progress has been achieved in developing new techniques for sputter deposition of oxide, nitride, and oxy-nitride thin films such as aluminum nitride, aluminum oxide, silicon nitride, silicon oxide, tantalum oxide, and tantalum oxynitride, among others. These films are being utilized in increasingly demanding ways that require increasing levels of control of the film properties. For example, piezoelectric aluminum nitride films used in various electroacoustic applications such as surface acoustic wave (SAW) and bulk acoustic wave (BAW) devices, thin film bulk acoustic resonators (FBAR) and microelectromechanical systems (MEMS) require dielectric films that are highly uniform, with low or no stress, and with a specific crystallographic orientation. Performance of these devices is substantially tied to the sputter technology that is used to deposit the aluminum nitride in terms of the film uniformity, texture, and stress. The ability to adjust the stress levels in thin films in sputter deposition equipment is advantageous for obtaining the necessary film properties for a wide range of devices for which the sputtering equipment can be used to fabricate.
Apparatus has been in use for some time for depositing sputtered atoms on a substrate to produce a layer of material defined by the sputtered atoms. The technique of sputtering material from a target to deposit on a substrate is commonly referred to as physical vapor deposition. Typical sputtering systems produce such a deposition by producing a glow discharge between an anode and a target, which acts as a cathode, to obtain an emission of sputtered atoms from the target. In most, if not all, sputtering equipment, a magnetic field is introduced in the vicinity of the sputter target to enhance the movement of electrons and subsequent ionization of the neutral gas and enable operation of the apparatus in the optimal sputter pressure regimes between 0.001 to 0.01 Torr within which sputtering yields from the target are optimal. An electric field applied in the vicinity of the target causes ions to bombard the surface of the sputter target. Electric fields can be applied using an alternating current power supply or a dc power supply. When, for example, an aluminum sputter target is bombarded with the ions of an inert gas such as argon, upon the application of the applied electric field, the target emits sputtered atoms of aluminum. The sputtered atoms travel to the substrate and become deposited on the substrate to produce a layer of the sputtered material. In systems in which an inert gas, or combination of inert gases, the method is commonly referred to as non-reactive physical vapor deposition. In non-reactive physical vapor deposition, the deposited layer is typically of the same stoichiometric composition as the target material.
An alternative to the non-reactive sputtering or non-reactive physical vapor deposition is reactive sputtering or reactive physical vapor deposition. In reactive sputtering, the deposited film is formed by plasma activated chemical reaction between a target material (metal, semiconductor, alloy) and a reactive gas such as oxygen or nitrogen which is mixed with an inert gas such as argon and introduced into a vacuum chamber equipped with a plasma source such as a magnetron.
Reactive sputtering methods are widely used in numerous electronic and surface engineering applications to produce thin dielectric films having certain functional characteristics. Silicon dioxide, for example, can be deposited using the reactive sputtering technique by introducing a mixture of argon and oxygen to a sputtering system equipped with a silicon sputter target. The oxygen in the mixture reacts with the sputtered silicon to form silicon dioxide. Similar reactions can occur using aluminum to form aluminum oxide, for example, or other materials which can react with oxygen to create a deposited film that incorporates oxygen with another element or combination of elements from the sputter target. The sputter target need not be made of a single element.
Alternatively, nitrogen can be mixed with an inert gas such as argon and introduced into the sputtering system to produce a reaction between the nitrogen and the sputtered target material to create a deposited film that incorporates nitrogen with another element or combination of elements from the sputter target. When aluminum is sputtered in the presence of nitrogen, for example, a deposited aluminum nitride layer can be formed on the substrate.
Two main approaches to power delivery for reactive sputtering are commonly employed: 1) pulsed dc, which is usually applicable for single target magnetron powering and 2) mid-frequency or alternating current (ac) powering, which is most effective for dual or other split cathode magnetron arrangements. Typical ac frequencies used in ac powered, split cathode configurations are in the range of 20-200 kHz.
The origin and evolution of intrinsic stress in thin films or structures of thin films can be viewed, for example, in terms of the processes responsible for the formation of the film microstructure. The type of stress, either compressive or tensile, and the magnitude of the stress have been shown to vary with the magnitude of the flux and energy of particles impinging on the growing film as reported by Windischmann, for example. The majority of magnetron sputtered metal films have a relatively low-density structure corresponding to zones 1 or T of the Structure Zone Model, as reported by Thornton, in which microvoids lead to the generation of tensile stress. Compressive stress is generated by an “atomic peening” mechanism whereby ions or accelerated neutrals from the plasma bombard the growing film creating interstitial atoms in the deposited film. Reactively sputtered oxide and nitride films often exhibit a tendency toward compressive stress due to a high concentration of reactive (for example, nitrogen or oxygen) gas atoms entrapped into the interstitial positions in the crystal lattice of the growing films.
The most effective methods for reducing the tensile stress in growing films are to employ ion assisted deposition and sputtering with substrate bias which enhances the ion bombardment of the film during deposition as reported by Chiu, et al. Ion bombardment during deposition results in argon entrapment and atomic peening, which promotes the displacement of surface atoms towards deeper positions in the bulk of the growing films leading to the filling of voids and atomic level vacancies and the formation of crystalline defects such as interstitial atoms.
In contrast, compressive stress can be reduced by restricting the generation of interstitial atoms by reducing the flux, by reducing the energy, or by reducing both the flux and the energy of energetic species arriving from the magnetron plasma discharge to the surface of the growing film. Compressive stress in the growing films can also be reduced by depositing the films at elevated temperature (higher adatom mobility allows interstitial atoms to be incorporated in the lattice) and by increasing the pressure of the sputter gas during the deposition of the films (sputtered atoms and ions experience more collisions with Ar atoms thus losing their energy before reaching the substrate).
In general, published results from investigations of stress in aluminum nitride films are consistent with known models. Este and Westwood reported that intrinsic stress in the films deposited from a planar aluminum target using rf discharge in argon/nitrogen mixtures changed drastically with increasing gas pressure from compressive −19 GPa to tensile +2.5 GPa. (In this context, a negative or minus stress is compressive and a positive or plus stress is tensile.) It was suggested that high compressive stress is due to bombardment of the film by energetic neutral nitrogen atoms reflected from the target, which is reduced as pressure is increased. Iriarte et al. completed a systematic study of the influence of the main process parameters on residual stress in fully textured polycrystalline aluminum nitride films deposited by a reactive pulsed dc magnetron. They revealed the effects of sputter gas pressure on stress through atom-assisted and atomic peening mechanisms. Dubois and Muralt showed that residual stress in aluminum nitride films deposited on a Pt electrode by reactive pulsed dc magnetron depended essentially on ion bombardment and on the sputtering pressure. Martin at al. found that aluminum nitride films deposited on Mo and Pt electrodes using pulsed dc sputter technology had inherent tensile stress, which might be reduced by depositing the aluminum nitride with a negative substrate bias.
There is sparse information in the literature related to the stress behavior of ac reactively sputtered highly textured aluminum nitride films. It was reported by Oshmyansky et al. that residual stress in the aluminum nitride films reactively deposited utilizing an ac powered magnetron with a dual-ring target configuration might be controlled partly by manipulating gas pressure and partly by manipulating magnetic field. Stress was changed from tensile +300 MPa to high compressive −1.3 GPa when the magnetic field strength was increased from 220 to 600 Gauss. It is necessary to point out that implementing aluminum nitride film stress control by manipulating the magnetic field influences the erosion profile of the sputtering target and, also, that it is a technically inconvenient method for industrial sputtering equipment. Adjustment of the pressure is also inconvenient since pressure can greatly affect other important characteristics of the sputter deposited films.
In view of the above, new methods are required to provide independent stress control in the formation of thin films, and in particular for thin films used in electro-acoustic device applications. It is the object of the present invention to provide a sputtering tool that enables control of the stress of sputter deposited films.